08 — Make wikilink stripping configurable

Priority: P3
Depends on: none
Size: S (30–45 min)
Files: sessionwiki/synth/pipeline.py, sessionwiki/cli.py, examples/sessions_config.json, tests/test_synth_pipeline.py
PR title: feat: make unresolved wikilink stripping configurable

Read first

sessionwiki/synth/pipeline.py, _strip_unresolved_wikilinks (around line 389) and synthesize_new_sessions (around line 900)
sessionwiki/cli.py, cmd_synthesize (around line 472)
examples/sessions_config.json, the synthesis block
AGENTS.md, the "Cross-link everything" hard rule

Problem

After every synthesis, _strip_unresolved_wikilinks replaces every [[Target]] whose page does not exist with plain text. This keeps link_integrity clean, but it is permanent. If the user creates the missing page later, the old source pages no longer link to it.

Some users prefer red links: they show what the wiki should contain.

Required change

Add a boolean option, default True to preserve current behavior.

Add a parameter to synthesize_new_sessions:

```python strip_unresolved_wikilinks: bool = True,


2. Guard the cleanup block:

 ```python
 if strip_unresolved_wikilinks and summary["synthesized"] > 0:
     ...

In cmd_synthesize, read the config:

```python strip_links = bool(synth_cfg.get("strip_unresolved_wikilinks", True))


 Pass it to `synthesize_new_sessions`.

4. Add the key to `examples/sessions_config.json`:

 ```json
 "strip_unresolved_wikilinks": true

Document the trade-off in one sentence in the synthesis block comment. Keep it short.

Do not change _strip_unresolved_wikilinks itself.

Tests

Add to tests/test_synth_pipeline.py:

A test with a fake backend that returns a body containing [[MissingPage]].
Run synthesize_new_sessions(..., strip_unresolved_wikilinks=False) against a temporary raw/sources tree.
Assert the written page still contains [[MissingPage]].
Run the same setup with strip_unresolved_wikilinks=True.
Assert the page contains MissingPage without brackets.

Use the existing _seed_raw helper or copy its fixture shape. Do not touch the real wiki.

Verification

.venv/bin/python -m pytest tests/test_synth_pipeline.py -q
.venv/bin/python -m pytest tests/ -q
.venv/bin/python -m sessionwiki lint

Expected: tests pass, lint reports 0 errors, and the default behavior is unchanged.

Acceptance criteria

[ ] synthesize_new_sessions accepts strip_unresolved_wikilinks.
[ ] The CLI reads synthesis.strip_unresolved_wikilinks.
[ ] Default is true; existing runs behave as before.
[ ] false preserves unresolved wikilinks.
[ ] Both test cases pass.
[ ] CHANGELOG.md updated.

Out of scope

Restoring links that were already stripped in existing pages.
Changing the link_integrity lint rule.
A per-project override.

Rollback

Remove the parameter and the config key. Default behavior does not change, so no data migration is needed.